PubSub support for multiple channels and context managers

[BenjaminYong]

Hi @DriverX , I was testing the sharded ssubscribe and subscribe function and noticed that PubSubs in your library are limited to a single channel, getting error:

• ValueError: Only one channel supported in cluster mode.

Are there plans to support multiple channels? Clients should be able to subscribe to multiple channels so long as they belong to the same slot. https://redis.io/commands/ssubscribe/.

• i.e.result = await redis_cluster.ssubscribe('{key}:123', '{key}:456'

Also, is it possible to open a Pub/Sub context manager to handle subscription messages?
In aioredis, this is possible like this example: https://aioredis.readthedocs.io/en/latest/examples/#pubsub.

i.e.

channels = ['{key}:123', '{key}:456']
async with self.redis_cluster.pubsub() as pubsub:
    await pubsub.ssubscribe(*channels)

    async for result in pubsub.listen():
        yield result

Many thanks!